Risley prisms are wedge-shaped glass prisms that enable variable beam deflection. Typically, Risley prisms are used as prism pairs in which one of the two prisms rotates to move from maximum to minimum beam deflection. Risley prisms are generally limited by lateral chromatic aberration, in which different wavelengths focus at the same image plane but at different transverse locations.
Typical star tracker and celestial navigation systems require large size, weight, and power (SWaP), and must be designed with very narrow field of view. There are also problems with beam steering mechanisms in such systems. For example, mechanical steering mechanisms typically add too much SWaP, and even then still may not achieve a required field of regard. Optical steering mechanisms in such systems typically introduce intolerable chromatic error or distortion.